1. Field of the Invention
The present invention relates to a toner for forming an electrophotographic image (which may also be referred to simply as “toner”, hereinafter), a method for manufacturing a toner for forming an electrophotographic image, an image forming method and a process cartridge.
2. Description of the Related Art
Recently, low-temperature fixing of a toner is desired in electrophotography. This is not only for energy saving to reduce energy required for fixing but also for demands to increase speed and image quality of an electrophotographic image forming apparatus.
In general, image quality decreases for a faster electrophotographic image forming apparatus. This involves various factors, and the most influential factor is an effect of deficient fixing in a fixing step.
In a fixing step, an unfixed toner image on a recording medium typified by paper becomes a fixed image by being fused on the recording medium due to heat and pressure. However, as system speed increases, the unfixed toner cannot obtain sufficient amount of heat in the fixing process. As a result, deficient fixing such as rough surface of the final toner image or image sticking effect called cold offset occurs, and it may become a defect image. Thus, when the system speed is increased, increasing the fixing temperature may be taken into consideration in order to maintain image quality. However, increasing the fixing temperature is not necessarily the best measure in view of side effects of the temperature which leaks from a fixing member on other processes in the image forming apparatus or acceleration of wearing speed of the fixing member.
Thus, improvement of fixing performance of a toner itself is desired in a high-speed image forming apparatus. Specifically, a toner which has sufficient fixing property at a lower temperature in a fixing step is desired.
Conventionally, various studies have been conducted to improve toner fixing property. For example, a method to control thermal characteristics of a resin itself typified by glass transition temperature (Tg) and softening temperature (T½) is heretofore known to improve the fixing performance of a toner. However, lowering the glass transition temperature (Tg) of a resin may degrade heat-resistant storage stability, and lowering the softening temperature (T½) by reducing the molecular weight of the resin, for example, causes problems such as hot offset. Thus, by controlling merely the thermal characteristics of the toner itself, a favorable toner with favorable low-temperature fixing property, heat-resistant storage stability and hot-offset resistance has not been obtained yet.
In order to respond to low-temperature fixing, use of a polyester resin which has superior low-temperature fixing property and relatively favorable heat-resistant storage stability has been tried in place of a styrene-acrylic resin which has been conventionally used heavily (for example, Japanese Patent Application Laid-Open (JP-A) Nos. 60-90344, 64-15755, 02-82267, 03-229264, 03-41470 and 11-305486 are referred to). Also, addition of a particular non-olefin crystalline polymer in a resin, where the polymer has a sharp-melting property at a glass transition temperature of the resin, has been tried in order to improve the low-temperature fixing property (JP-A No. 62-63940). However, it is not necessarily optimizing molecular structure or molecular weight.
Also, a technology to improve fixing property by employing a crystalline polyester which has a sharp-melting property similarly to the particular non-olefin crystalline polymer has been proposed (Japanese Patent (JP-B) No. 2931899 and JP-A No. 2001-222138). However, a toner which uses the crystalline polyester described in JP-B No. 2931899 has a low acid value and hydroxyl value of 5 or less and 20 or less, respectively, and thus it does not have sufficient low-temperature fixing property since the affinity between paper and the crystalline polyester is low.
Regarding a toner which uses the crystalline polymer described in JP-A No. 2001-222138, a molecular weight or existing condition of the crystalline polyester is not optimized in the toner as a final product. Thus, the toner which uses the crystalline polymer described in JP-A No. 2001-222138 does not necessarily and sufficiently provide superior low-temperature fixing property and heat-resistant storage stability which comes from the crystalline polyester after production of the toner. Also, no countermeasure has been taken with respect to hot-offset resistance, and a temperature range for fixing favorable image is not necessarily ensured.
Also, a technology to form a sea-island phase-separation structure of a crystalline polyester and a non-crystalline polyester, which are incompatible with each other, has been proposed (JP-A No. 2004-46095). However, a toner of this proposal uses three types of resins including a crystalline polyester resin, and in order to maintain the sea-island structure of the crystalline polyester resin in this technology, dispersion diameter of the crystalline polyester resin becomes too large. This interferes with heat-resistant storage stability, or transfer failure occurs in transferring because electrical resistance is too low. As a result, a finally obtained image may become rough.
Also, JP-A No. 2007-33773 proposes a technology to control an existing state of a crystalline polyester resin by defining an amount of an endothermic peak appearing at a heat absorption side in a DSC curve measured by a differential scanning calorimeter so as to exert a significant effect of the crystalline polyester resin and to provide a toner with a low-temperature fixing property and heat-resistant storage stability. However, this proposal assumes to use a resin having a relatively high softening temperature as a non-crystalline polyester resin used in combination with the crystalline polyester resin, and thus the crystalline polyester resin bears a role of low-temperature fixing property. Therefore, an amount of the crystalline polyester resin used increases inevitably, and a risk of degraded heat-resistant storage stability increases due to miscibility with the non-crystalline resin.
Also, JP-A No. 2005-338814 proposes a technology that a toner includes a large amount of crystalline polyester resin. However, this proposal has a problem that heat-resistant storage stability degrades due to miscibility with a non-crystalline resin because the content of the crystalline polyester resin is very large.
Also, JP-B No. 4118498 proposes a technology to define a peak and a half-value width of a molecular weight distribution of a toner and an amount of chloroform insoluble matter or to use two or more types of resins having different softening temperatures as a binder resin. In this proposal, however, a crystalline polyester resin is not used, and low-temperature fixing property is insufficient compared to a toner with a crystalline polyester resin.
Conventionally, as a developing apparatus using a two-component developer including a toner and a magnetic carrier, an apparatus having a structure illustrated in FIG. 1 has been know. In the developing apparatus 4 illustrated in FIG. 1, a path to supply a developer to a developing roller 5 as a developer bearing member and a path to stir the developer are separately arranged, and the developer is circulated by conveying it in the two paths in opposite directions.
In the developing apparatus 4 illustrated in FIG. 1, the path to supply the developer to the developing roller 5 and a path to collect the developer which has been supplied to the developing roller 5 and passed the developing region are in common. Thus, there is a problem that the developer which has been supplied to the developing roller 5 has a lower toner concentration at a more downstream side in a conveying direction of the path which supplies the developer to the developing roller 5. When the developer supplied to the developing roller 5 has a decreased toner concentration, an image density in development decreases as well. In FIGS. 1, 10, 11, 401 and 403 represent a stirring path, a mixing auger, a mixing auger and a partition wall, respectively.
JP-B No. 3127594 and JP-A No. 11-167260 propose a developing apparatus as a solution to such a problem, where an auger for supplying a developer to a developing roller and an auger for collecting a developer after development are arranged in separate developer paths. Hereinafter, structures of the developing apparatuses described in JP-B No. 3127594 and JP-A No. 11-167260, respectively, are explained.
FIG. 2 illustrates a developing apparatus described in JP-B No. 3127594.
In a developing apparatus 4 illustrated in FIG. 2, a supplying path 9 for supplying a developer to a developing roller 5 and a collecting path 7 for collecting the developer which has passed a developing region are separately arranged. In FIG. 2, 1 represents a photoconductor; 6, 8 and 209 represent a mixing auger; 16 represents a developer regulating member; and 403 represents a partition wall.
In the developing apparatus 4, the developer having passed through the developing region is not mixed into the supplying path 9 because it is sent to the collecting path 7. By this, the toner concentration of the developer in the supplying path 9 does not vary, and the developer supplied to the developing roller 5 remains constant.
However, even though a toner is replenished for an appropriate toner concentration, the developer is not sufficiently stirred because the developer sent to the collecting path 7 is immediately supplied to the supplying path. Thus, there is a problem of an uneven image density or density decrease in development. Such problems are more prominent for an image having a high printing rate because the toner concentration of the collected developer decreases.
Next, a developing apparatus described in JP-A No. 11-167260 is illustrated in FIG. 3.
A developing apparatus illustrated in FIG. 3, a supplying path 9 for supplying a developer to a developing roller 5 and a collecting path 7 for collecting the developer which has passed a developing region are separately arranged. Further, it is equipped with a stirring path 10 which stirs the developer conveyed to the most downstream side of the supplying path 9 and a collected developer conveyed to the most downstream side of the collecting path 7 and conveys the developer in a direction opposite to the supplying path 9.
In the developing apparatus 4, the developer having passed through the developing region is not mixed into the supplying path 9 because it is sent to the collecting path 7. By this, the toner concentration of the developer in the supplying path 9 does not vary, and the developer supplied to the developing roller 5 remains constant. In FIGS. 3, 6, 8 and 11 represent mixing augers, respectively; 16 represents a developer regulating member; and 404 and 405 represent partition walls respectively.
Further, it does not supply the collected developer immediately to the supplying path 9; rather, it supplies the developer to the supplying path 9 after stirring it in the stirring path 10. Thus, it is possible to supply the developer again to the supplying path in a condition that the developer having passed through the supplying path 9 without being used in development and the collected developer are stirred. Accordingly, a technique to prevent uneven image density and decrease in image density in development, which was the problem of the developing apparatus explained with FIG. 2 is demonstrated.
However, the technologies described in JP-B No. 3127594 and JP-A No. 11-167260 aim at stabilizing the concentration of an output image, and they do not aim at improving overall quality of a system for forming an electrophotographic image forming system such as low-temperature fixing property, hot-offset resistance and a storage stability of a toner.